Typical aircraft turbofan jet engines include a fan that draws and directs a flow of air into a nacelle and into and around an engine core. The nacelle surrounds the engine core and helps promote the laminar flow of air around the core. The flow of air that is directed into the engine core is initially passed through a compressor that increases the air flow pressure, and then through a combustor where the air is mixed with fuel and ignited. The combustion of the fuel and air mixture causes a series of turbine blades at the rear of the engine core to rotate, and to drive the engine's rotor and fan. The high-pressure exhaust gases from the combustion of the fuel and air mixture are thereafter directed through an exhaust nozzle at the rear of the engine.
Bypass flow is air that is directed around the engine core. In turbofan engines, the bypass flow typically provides the main thrust for an aircraft. The bypass flow also can be used to help slow a landed aircraft. Thrust reversers mounted in the nacelle structure selectively reverse the direction of the bypass flow to generate reverse thrust. During normal engine operation, the bypass flow may or may not be mixed with the engine core exhaust before exiting the engine assembly.
Several turbofan engine parameters are important to optimize design characteristics and performance. An engine's bypass ratio (BPR) is the ratio of the air mass that passes through the engine's fan duct to that passing through the engine core. Higher BPR engines can be more efficient and quiet than lower BPR engines. In general, a higher BPR results in lower average exhaust velocities and less jet noise at a specific thrust rating. A turbofan engine's performance is also affected by the engine's fan pressure ratio (FPR). FPR is the ratio of the air pressure at the engine's fan nozzle exit to the pressure of the air entering the fan. The lower the FPR, the lower the exhaust velocity, and the higher an engine's propulsive efficiency. Reducing an engine's FPR can reach a practical limit, however, as a low FPR can cause engine fan stall, blade flutter or compressor surge under certain operating conditions.
One solution to these problems includes varying the fan nozzle exit area of a high-BPR engine during operation to optimize engine performance under various flight conditions. By selectively varying the fan nozzle's exit area, an engine's bypass flow characteristics can be adjusted to match a particular flight condition. Unfortunately, prior variable area nozzle systems typically have been heavy, expensive and somewhat complex in their structure and operation, and generally require the coordinated movement of multiple components that employ complex drive mechanisms.
Accordingly, a need exists for a variable area nozzle assembly for turbofan aircraft engine that promotes a cost effective, simple and efficient operation for control of engine output under certain flight conditions.